So far there have been proposed a number of methods and devices for compensating the decrease of net area of total absorption peaks in the amplitude spectra, due either to "off" time or to pulse superposition.
To compensate the loss due to "off" time in all conventional spectrometric analyzers, means are provided for measuring the net time, called also operating or "on" time of the analyzer, throughout which the input of the analyzer is opened to accept pulses from a detector responsive to a stream of particles, such as neutrons, from the samples. The duration of each measurement is defined by the "on" time and the diminution of the net area of the total absorption peaks, due to the "off" time, and is compensated through virtual prolongation of the measurement duration.
A disadvantage of this method resides in the pulse losses due to the "off" time of electronic devices connected ahead of the analog-to-digital converter of the analyzer and the detector.
A further method has become known for compensating the decrease of the net peak area by measuring both the standard sample and the test sample simultaneously with another radioactive isotope, serving as an internal reference, with equal quantities used in both measurements. The ratio of the net areas of the peak of total absorption of this isotope, obtained after both measurements, defines the decrease of the net area of the test sample as compared to that of the standard sample. This method has the following disadvantages: the isotope chosen for the internal reference should be of an element absent in the test sample, which requires either preliminary knowledge of the chemical composition of the specimen to be tested or performace of separate tests for qualitative determination of this composition; a large number of various isotopes should be available in order to find out a proper internal reference for each test; the internal reference itself charges additionally the detector system and prolongs the total duration of the tests; to obtain the final results, additional calculations are required along with introduction of a corrective factor; the correction performed produces a certain error due both to the statistical distribution of the pulses from the internal reference and to the inability to distinguish them from pulses of like amplitude for the test sample and the background.
Another method is known wherein the testing of standard samples is carried out under conditions of pulse loading of the detector and analyzer duplicating those existing with the test sample. For this purpose a mixture of radioactive isotopes, of a quantitive composition close to that of the test sample, is tested along with the standard sample. The disadvantage of this method is its high labor cost and waste of time for choosing and testing the isotope mixture several times in order to achieve similarity with the composition of the test sample, this mixture varying with different samples, as well as the limited efficiency of the method and its inability to evaluate the error, because no perfect identity between the sample and the isotope can be achieved and the significance of the residual difference cannot be estimated quantitatively.
Electronic rejectors have also been proposed which separate the pulses whose amplitue is changed because of superposition before the pulses are transmitted to the analyzer. As soon as the rejectors are switched on, the change in the shapes of the peaks of total absorption disappears. The disadvantage of the rejectors, along with their being too complex and too expensive, is their inability to estimate quantitatively the separate pulses from each peak and therefore they cannot be applied in quantitative analyses.